1. Field of Invention
This invention relates generally to the field of fabrication with nanoparticles and, more particularly, to the low temperature maskless fabrication of functional structures with nanoparticles and, most particularly, to the nanofabrication of passive and active electronic structures and devices.
2. Description of Prior Art
The development of direct printing of functional material has gained significant interest as an alternative to conventional integrated circuit (IC) processing, particularly for applications to low-cost flexible electronics. While conventional lithographic processes are well developed for the patterning of inorganic microelectronics, flexible polymer surfaces are often incompatible with typical photoresists, etchants and developers used in conventional IC processes.
In addition, other challenges exist in attempting to utilize conventional IC processing techniques to produce electronic components on flexible, typically polymeric, substrates. Conventional IC processes are typically multi-step processes that require high processing temperatures and generate toxic wastes which combine to add to the cost.
One approach to overcoming these problems is to employ drop-on-demand (DOD) inkjet printing. Since inkjet printing is an additive process, many of the problems encountered with conventional IC processing technologies can be overcome in a cost-effective manner. The fully data-driven and maskless nature of DOD inkjet printing typically allows increased processing versatility compared to other direct printing methods. The material is typically deposited on the surface along with a carrier solution, typically by means of a piezo-electrically driven micro-capillary tube, which provides enhanced processing flexibility for choosing both the material and the substrate. However, inkjet printing of electronic devices and structures utilizing nanoparticles has challenges of its own.
The term “nanoparticles” as used herein designates particles having a diameter less than approximately one micron (micron=1 μm=10−6 meter), typically much less than one micron or well into the submicron range. It is known in the art that such particles exhibit thermophysical properties substantially different from those of the corresponding bulk materials. In particular, the melting point typically decreases substantially for particles having diameters below approximately 100 nm (nm=nanometer=10−9 meter), and in particular below approximately 10 nm. For example, nanoparticles of gold having diameters of approximately 2.5 nm show a melting point of approximately 300° C. to 400° C., whereas the melting point of bulk gold is 1063° C. For economy of language, we refer to fabrication of structures and devices from nanoparticles as nanofabrication.
In WO 00/10197 this effect of depressed melting point is exploited for producing copper structures on a semiconductor wafer at low temperatures. A suspension of copper nanoparticles in a liquid is applied to a semiconductor chip. After evaporation of the solvent, the nanoparticles are concentrated in recesses in the wafer surface and the wafer is heated above the particles' melting point to sinter or melt them. This method takes advantage of the comparatively low melting point of the particles, but it requires the presence of suitable recesses in the surface of the substrate.
Published Japanese patent application JP2000014101 describes a method for forming structures by focusing a laser beam into a storage tank containing a suspension of superfine particles. This method requires a large amount of the suspension and is therefore expensive.
Szezech et al in IEEE Transactions on Electronics Packaging Manufacturing, Vol. 25, No. 1, pp. 26-33 (2002) have constructed fine conductor lines by drop-on-demand jet printing of nanoparticles suspended in solution followed by evaporation and sintering of the structure by inserting the entire structure into an oven maintained at moderate temperature (300° C.).
Thus, in view of the foregoing a need exists in the art for simple and efficient methods, materials and equipment for producing structures from nanoparticles without the need for recesses on the surface in which the particles collect, or for heating the entire substrate.
Additionally, since many substrates on which it would be desirable to fabricate passive and active electronic components are incompatible with the high temperatures, photoresists, etchants or developers typically employed in subtractive lithographic processes conventionally used to fabricate ICs, a further need exists in the art for systems, materials and methods that comprise additive processes for the production of ICs.
While inkjet processes achieves certain advantages as noted above, typical inkjet processes have the disadvantage of having coarser resolution than that achievable with conventional IC processing. The resolution of inkjet processing is governed chiefly by the nozzle diameter, typically about the same as the droplet diameter, and the variation of the droplet flight to the substrate, and the droplet spreading on the substrate. Thus, a further need exists in the art to achieve inkjet fabrication of electronic structures and/or devices with improved resolution.